1. Field of the Invention
The present invention relates to materials and methods for protecting surfaces subject to frictional forces, heat, and corrosion, and more particularly, to wear-resistant coatings that can be applied to piston rings and cylinder liners of internal combustion engines.
2. Discussion
A power cylinder assembly of an internal combustion engine generally comprises a reciprocating piston disposed within a cylindrical cavity of an engine block. One end of the cylindrical cavity is closed while another end of the cylindrical cavity is open. The closed end of the cylindrical cavity and an upper portion or crown of the piston, define a combustion chamber. The open end of the cylindrical cavity permits oscillatory movement of a connecting rod, which joins a lower portion of the piston to a crankshaft, which is partially submerged in an oil sump. The crankshaft converts linear motion of the piston—resulting from combustion of fuel in the combustion chamber—into rotational motion.
The power cylinder assembly typically includes one or more piston rings and a cylindrical sleeve or cylinder liner, which is disposed within the engine block and forms the side walls of the cylindrical cavity. The piston rings are disposed in grooves formed in the lateral walls of the piston, and extend outward from the piston into an annular space delineated by the piston wall and the cylinder liner. During movement of the piston within the cylindrical cavity, the piston rings bear against the cylinder liner. The piston rings have two main functions. First, they inhibit gas flow from the combustion chamber into the oil sump through the annular space between the piston and the cylinder liner. Second, they minimize oil flow from the oil sump into the combustion chamber.
To improve their durability, wear and scuff resistance, the piston rings, and in some cases the cylinder liner, are coated with relatively hard materials such as chromium hard plate and alloys containing chromium carbide. Although such coatings have met with considerable success, they have been found inadequate for newer engine technologies, including diesel engines employing exhaust gas recirculation (EGR).